An experimental study of hydrogen autoignition in a turbulent co-flow of heated air

The autoignition behaviour of hydrogen in a turbulent co-flow of heated air at atmospheric pressures was examined experimentally. Turbulent flows of air, with temperatures up to 1015 K and velocities up to 35 m/s, were set up in an optically accessible tube of circular cross-section. The fuel, pure or diluted with nitrogen, was continuously injected along the centreline of the tube, with velocities equal to or larger than those of the air, and temperatures that were lower. The fuel mixing patterns hence obtained were akin to diffusion from a point source or to an axisymmetric jet within a co-flow. For a relatively wide range of temperatures and velocities, a statistically steady condition of randomly occurring autoignition kernels was observed, whose axial location was measured by hydroxyl radical chemiluminescence. The probability density function of autoignition location was sharp enough to allow the accurate determination of a minimum autoignition length and smooth enough to allow the mean and variance to be calculated. It was found that both autoignition lengths increased with the air velocity and decreased with the air temperature, as expected. An estimate of the residence time up to autoignition showed that the autoignition delay times increased with the air velocity for the same temperature, suggesting a delaying effect of the turbulence on autoignition. The connection between these findings and previous experimental and direct numerical simulation studies is discussed.     

Dynamics of triple-flames in ignition of turbulent dual fuel mixture: A direct numerical simulation study

Pilot-ignited dual fuel combustion involves a complex transition between the pilot fuel autoignition and the premixed-like phase of combustion, which is challenging for experimental measurement and numerical modelling, and not sufficiently explored. To further understand the fundamentals of the dual fuel ignition processes, the transient ignition and subsequent flame development in a turbulent dimethyl ether (DME)/methane-air mixing layer under diesel engine-relevant conditions are studied by direct numerical simulations (DNS). Results indicate that combustion is initiated by a two-stage autoignition that involves both low-temperature and high-temperature chemistry. The first stage autoignition is initiated at the stoichiometric mixture, and then the ignition front propagates against the mixture fraction gradient into rich mixtures and eventually forms a diffusively-supported cool flame. The second stage ignition kernels are spatially distributed around the most reactive mixture fraction with a low scalar dissipation rate. Multiple triple flames are established and propagate along the stoichiometric mixture, which is proven to play an essential role in the flame developing process. The edge flames gradually get close to each other with their branches eventually connected. It is the leading lean premixed branch that initiates the steady propagating methane-air flame. The time required for the initiation of steady flame is substantially shorter than the autoignition delay time of the methane-air mixture under the same thermochemical condition. Temporal evolution of the displacement speed at the flame front is also investigated to clarify the propagation characteristics of the combustion waves. Cool flame and propagation of triple flames are also identified in this study, which are novel features of the pilot-ignited dual fuel combustion.     

Conditional Moment Closure (CMC) applied to autoignition of high pressure methane jets in a shock tube

Autoignition of non-premixed methane–air mixtures is investigated using first-order Conditional Moment Closure (CMC). Turbulent velocity and mixing fields simulations are decoupled from the CMC calculations due to low temperature changes until ignition occurs. The CMC equations are cross-stream averaged and finite differences are applied to discretize the equations. A three-step fractional method is implemented to treat separately the stiff chemical source term. Two detailed chemical kinetics mechanisms are tested as well as two mixing models. The present results show good agreement with published experimental measurements for the magnitude of both ignition delay and kernel location. The slope of the predicted ignition delay is overpredicted and possible sources of discrepancy are identified. Both scalar dissipation rate models produce comparable results due to the turbulent flow homogeneity assumption. Further, ignition always occurs at low scalar dissipation rates, much lower than the flamelet critical value of ignition. Ignition is found to take place in lean mixtures for a value of mixture fraction around 0.02. The conditional species concentrations are in qualitative agreement with previous research. Homogeneous and inhomogeneous CMC calculations are also performed in order to investigate the role of physical transport in the present autoignition study. It is found that spatial transport is small at ignition time. Predicted ignition delays are shown to be sensitive to the chemical kinetics. Reasonable agreement with previous simulations is found. Improved formulations for the mixing model based on non-homogeneous turbulence are expected to have an impact.     

Transported scalar PDF calculations of autoignition of a hydrogen jet in a heated turbulent co-flow

The joint scalar PDF method, as implemented in FLUENT, was used to simulate the autoignition of a jet of hydrogen in a turbulent co-flow of heated air. While the autoignition phenomenon is intermittent in the experiment, ensemble-averaged data on the effect of the flow on ignition length are available, which enables us to compare them with the steady state calculations.Results of sensitivity tests showed that the choice of chemical mechanism affects the calculation more than the mixing model and model constants. Further calculations for different initial conditions (i.e. temperature and velocity of the jet T _jet and U _jet and the co-flow T _air and U _air) have been done using a set of parameters selected after the sensitivity study. Scatter plots and conditional scalar profiles confirmed that the ignition is always initiated in lean mixture fractions. The ignition length was predicted with good accuracy for the case of U _jet>U _air but not so well for the case of U _jet≈U _air. For the equal velocity case, increasing the velocity resulted in delayed autoignition time (defined as the ignition length divided by the mean velocity), in agreement with the experimental trend. The results give credence to the use of the joint scalar PDF method for autoignition in non-premixed flows.     

Coupling of turbulence on the ignition of multicomponent sprays

This study examines the effect of turbulence on the ignition of multicomponent surrogate fuels and its role in modifying preferential evaporation in multiphase turbulent spray environments. To this end, two zero-dimensional droplet models are considered that are representative of asymptotic conditions of diffusion limit and the distillation limit are considered. The coupling between diffusion, evaporation and combustion is first identified using a scale analysis of 0D homogeneous batch reactor simulations. Subsequently, direct numerical simulations of homogeneously dispersed multicomponent droplets are performed for both droplet models, in decaying isotropic turbulence and at quiescent conditions to examine competing time scale effects arising from evaporation, ignition and turbulence. Results related to intra-droplet transport and effects of turbulence on autoignition and overall combustion are studied using an aviation fuel surrogate. Depending on the characteristic scale, it is shown that turbulence can couple through modulation of evaporation time or defer the ignition phase as a result of droplet cooling or gas-phase homogenization. Both preferential evaporation and turbulence are found to modify the ignition delay time, up to a factor of two. More importantly, identical droplet ignition behavior in homogeneous gas phase can imply fundamentally different combustion modes in heterogeneous environments.     

Simulation of spray development and turbulent combustion processes in low and high speed diesel engines by the CMC-ISR model

Simulation is performed to analyse the characteristics of turbulent spray combustion in conventional low and high speed diesel engine conditions. Turbulence–chemistry interaction is resolved by the Conditional Moment Closure (CMC) model in the spatially integrated form of an Incompletely Stirred Reactor (ISR). After validation against measured pressure traces, characteristic length and time scales and dimensionless numbers are estimated at the locations of sequentially injected fuel groups. Conditional flame structures are calculated for sequentially evaporated fuel groups to consider different available periods for ignition chemistry. Injection overlaps the combustion period in the high rpm engine, while most combustion occurs after injection and evaporation are complete in the low rpm engine. Ignition occurs in rich premixture with the initial peak temperature at the equivalence ratio around 2–4 as observed in Dec [2]. It corresponds to the most reactive mixture fraction of the minimum ignition delay for the given mixture states. Combustion proceeds to lean and rich sides in the mixture fraction space as a diffusion process by turbulence. The mean scalar dissipation rates (SDRs) are lower than the extinction limit to show stability of diffusion flames throughout the combustion period.     

Predicting the ignition delay of turbulent methane jets using Conditional Source-term Estimation

A predictive simulation of the autoignition process of non-premixed methane in a turbulent jet configuration was performed. Closure for the chemical source-term was obtained using Conditional Source-term Estimation with Laminar Flamelet Decomposition (CSE-LFD). The ambient oxidizer conditions – the high pressure and moderate temperatures characteristic of compression ignition engines – were chosen with the intent to validate the combustion model used under engine-relevant conditions. Validation was obtained by comparison of the predicted ignition delay to experimental results obtained from a shock-tube facility at several initial temperatures. Overall, the combination of full chemistry that has been carefully tuned to predict autoignition of premixed methane–air mixtures under similar temperature/pressure conditions with the CSE-LFD model is able to successfully predict the autoignition delay time of methane–air jets well within the scatter in the experimental data.     

Identification of combustion mode under MILD conditions using Chemical Explosive Mode Analysis

Direct Numerical Simulations (DNS) data of Moderate or Intense Low-oxygen Dilution (MILD) combustion are analysed to identify the contributions of the autoignition and flame modes. This is performed using an extended Chemical Explosive Mode Analysis (CEMA) which accounts for diffusion effects allowing it to discriminate between deflagration and autoignition. This analysis indicates that in premixed MILD combustion conditions, the main combustion mode is ignition for all dilution and turbulence levels and for the two reactant temperature conditions considered. In non-premixed conditions, the preponderance of the ignition mode was observed to depend on the axial location and mixture fraction stratification. With a large mixture fraction lengthscale, ignition is more preponderant in the early part of the domain while the deflagrative mode increases further downstream. On the other hand, when the mixture fraction lengthscale is small, sequential autoignition is observed. Finally, the various combustion modes are observed to correlate strongly with mixture fraction where lean mixtures are more likely to autoignite while stoichiometric and rich mixtures are more likely to react as deflagrative structures.     

On laminar premixed flame propagating into autoigniting mixtures under engine-relevant conditions

Usually premixed flame propagation and laminar burning velocity are studied for mixtures at normal or elevated temperatures and pressures, under which the ignition delay time of the premixture is much larger than the flame resistance time. However, in spark-ignition engines and spark-assisted compression ignition engines, the end-gas in the front of premixed flame is at the state that autoignition might happen before the mixture is consumed by the premixed flame. In this study, laminar premixed flames propagating into an autoigniting dimethyl ether/air mixture are simulated considering detailed chemistry and transport. The emphasis is on the laminar burning velocity of autoigniting mixtures under engine-relevant conditions. Two types of premixed flames are considered: one is the premixed planar flame propagating into an autoigniting DME/air without confinement; and the other is premixed spherical flame propagating inside a closed chamber, for which four stages are identified. Due to the confinement, the unburned mixture is compressed to high temperature and pressure close to or under engine-relevant conditions. The laminar burning velocity is determined from the constant-volume propagating spherical flame method as well as PREMIX. The laminar burning velocities of autoigniting DME/air mixture at different temperatures, pressures, and autoignition progresses are obtained. It is shown that the first-stage and second-stage autoignition can significantly accelerate the flame propagation and thereby greatly increase the laminar burning velocity. When the first-stage autoignition occurs in the unburned mixture, the isentropic compression assumption does not hold and thereby the traditional method cannot be used to calculate the laminar burning velocity. A modified method without using the isentropic compression assumption is proposed. It is shown to work well for autoigniting mixtures. Besides, a power law correlation is obtained based on all the laminar burning velocity data. It works well for mixtures before autoignition while improvement is still needed for mixtures after autoignition.     

Direct numerical simulations of exhaust gas recirculation effect on multistage autoignition in the negative temperature combustion regime for stratified HCCI flow conditions by using H2O2 addition

Direct numerical simulations (DNSs) of a stratified flow in a homogeneous compression charge ignition (HCCI) engine are performed to investigate the exhaust gas recirculation (EGR) and temperature/mixture stratification effects on the autoignition of synthetic dimethyl ether (DME) in the negative temperature combustion region. Detailed chemistry for a DME/air mixture is employed and solved by a hybrid multi-time scale (HMTS) algorithm to reduce the computational cost. The effect of to mimic the EGR effect on autoignition are studied. The results show that adding enhances autoignition by rapid OH radical pool formation (34–46% reduction in ignition delay time) and changes the ignition heat release rates at different ignition stages. Sensitivity analysis is performed and the important reactions pathways affecting the autoignition are specified. The DNS results show that the scales introduced by thermal and mixture stratifications have a strong effect after the low temperature chemistry (LTC) ignition especially at the locations of high scalar dissipation rates. Compared to homogenous ignition, stratified ignitions show similar first autoignition delay times, but 18% reduction in the second and third ignition delay times. The results also show that molecular transport plays an important role in stratified low temperature ignition, and that the scalar mixing time scale is strongly affected by local ignition in the stratified flow. Two ignition-kernel propagation modes are observed: a wave-like, low-speed, deflagrative mode and a spontaneous, high-speed, ignition mode. Three criteria are introduced to distinguish these modes by different characteristic time scales and Damkhöler numbers using a progress variable conditioned by an ignition kernel indicator. The low scalar dissipation rate flame front is characterized by high displacement speeds and high mixing Damkhöler number. The proposed criteria are applied successfully at the different ignition stages and approximate characteristic values are identified to delineate between the different ignition propagation modes.     

Evaporating droplets in turbulent reacting flows

Three-dimensional direct numerical simulations are carried out to determine the effects of turbulence on the preferential segregation of an evaporating spray and then to study the evolution of the resulting mixture fraction topology and propagating flame. First, the mixing between an initially randomly dispersed phase and the turbulent gaseous carrier phase is studied with non-evaporating particles. According to their inertia and the turbulence properties, the formation of clusters of particles is analyzed (formation delay, cluster characteristic size and density). Once the particles are in dynamical equilibrium with the surrounding turbulent flow, evaporation is considered through the analysis of the mixture fraction evolution. Finally, to mimic ignition, a kernel of burnt gases is generated at the center of the domain and the turbulent flame evolution is described.     

一种低湍流扬尘方法的实验研究

对一种新型扬尘方法在垂直管道中形成的扬尘湍流特性进行了测量,在此基础上,观察和测量了玉米粉尘火焰向上传播的过程,讨论了湍流对火焰特性的影响。新方法产生的扬尘湍流强度相当低,随时间衰减缓慢,扬尘湍流的积分尺度随着时间增大,约为2 cm到3 cm。实验中观察到两种粉尘火焰:湍流火焰和层流火焰,火焰形态转变对应的点火延迟时间约等于1．1 s,即粉尘云湍流运动强度为10 cm／s,湍流火焰传播速度明显大于层流火焰。     

An experimental investigation on pre-ignition phenomena: Emphasis on the role of turbulence

Pre-ignition is an undesirable ignition event that affects chemical kinetic measurements in chemical reactors. Meanwhile, it appears randomly in engineering systems and is highly relevant to the soft knock or much stronger and detrimental super-knock in modern downsized engines. Currently its origins are still not fully understood. In this study, the role of turbulence in pre-ignition phenomena was experimentally investigated using a novel rapid compression machine. Different turbulent flow fields were achieved through calibrated orifice plates. Stoichiometric isooctane/air mixtures were tested under engine-relevant conditions in a target pressure range of 15–50 bar and a temperature range of 720–860 K. Useful insights into pre-ignition mechanism were obtained by combining instantaneous pressure acquisition with simultaneously recorded high-speed imaging. The experimental results demonstrate that owning to turbulent mixing with colder boundary layers, ignition timing is delayed when compared to ideal homogeneous compression ignition scenarios. However, pre-ignition phenomena can still be observed and become pronounced at lower target pressures with longer ignition delays. Moreover, pre-ignition formation can be characterized by single or multiple spherical flame kernels, distributed discretely inside core mixture or at near-wall regions. Different from the auto-ignition scenarios dominated by the chemical reactivity of test mixture, these pre-ignition flame kernels feature standard deflagration propagation. Finally, a dimensionless scaling analysis shows that pre-ignition formation is closely associated with turbulent length scale and laminar flame thickness.     

Analysis of auto-ignition of heated hydrogen–air mixtures with different detailed reaction mechanisms

Auto-ignition processes of hydrogen, diluted with nitrogen, in heated air are numerically investigated by means of an unsteady laminar flamelet approach in mixture fraction space. The focus is on the auto-ignition delay time and the most reactive mixture fraction as obtained with five chemical mechanisms. Two strongly different levels of dilution, corresponding to experiments in the open literature, are considered. This concerns low-temperature chemistry at atmospheric pressure. The temperature of the air stream is much higher than the temperature of the fuel stream in the cases under study. We extensively investigate the effect of the co-flow temperature, the conditional scalar dissipation rate and the resolution in mixture fraction space for one case. With respect to the conditional scalar dissipation rate, we discuss the Amplitude Mapping Closure (AMC) model with imposed maximum scalar dissipation rate at mixture fraction equal to 0.5, as well as a constant conditional scalar dissipation rate value over the entire mixture fraction value range. We also illustrate that an auto-ignition criterion, based on a temperature rise, leads to similar results as an auto-ignition criterion, based on OH mass fraction, provided that the hydrogen is not too strongly diluted.     

Effects of temperature and equivalence ratio on the ignition of n-heptane fuel spray in turbulent flow

Direct numerical simulations were performed to study the autoignition process of n-heptane fuel spray in a turbulent field. For the solution of the carrier gas fluid, the Eulerian method is employed, while for the fuel droplets, the Lagrangian method is used. Droplets are initialized at random locations in a two-dimensional isotropic turbulent field. A chemistry mechanism for n-heptane with 44 species and 112 reactions was adopted to describe the chemical reactions. Three cases with the same initial global equivalence ratio (0.5) and different initial gas phase temperatures (1100, 1200, and 1300 K) were simulated. In addition, two cases with initial global equivalence ratios of 1.0 and 1.5 and initial temperature 1300 K were simulated to examine the effect of equivalence ratio. Evolution of temperature, species mass fraction, reaction rate, and the joint PDF of temperature and equivalence ratio are presented. Effects of the initial gas temperature and equivalence ratio on vaporization and ignition are discussed. A correlation was derived relating ignition delay times to temperature and equivalence ratio. It was confirmed that with the increase of initial temperature, the autoignition occurs earlier. With the increase of the initial equivalence ratio, however, autoignition occurs later due to a larger decrease in gas phase temperature caused by fuel droplet evaporation. The results obtained in this study are expected to be constructive in understanding fuel spray combustion, such as that in homogeneous charge compression ignition systems.     

Experimental and modeling study of the autoignition characteristics of commercial diesel under engine-relevant conditions

Knowledge of the autoignition characteristics of diesel fuels is of great importance for understanding the combustion performance in engines and developing surrogate fuels. Here ignition delays of China's stage 6 diesel, a commercial fuel, were measured in a heated rapid compression machine (RCM) under engine-relevant conditions. Gas-phase autoignition experiments were carried out at equivalence ratios ranging from 0.37 to 1.0, under compressed pressures of 10, 15, and 20?bar, and within a temperature range of 685–865?K. In all investigated conditions, negative temperature coefficient (NTC) behavior of the total ignition delays is observed. The autoignition of the diesel fuel exhibits pronounced two-stage characteristics with strong low-temperature reactivity. Experimental results indicate that the total ignition delays shorten with increasing compressed pressure, oxygen mole fraction and fuel mole fraction. The first-stage ignition delays are mainly controlled by compressed temperature and also affected by oxygen mole fraction and compressed pressure but show a very weak dependence on fuel mole fraction. Correlations describing the first-stage ignition delay and the total ignition delay were proposed to further clarify the ignition delay dependence on the multiple factors. Additionally, it is found that the newly measured ignition delays well coincide with and complement the diesel ignition data in the literature. A recently developed diesel mechanism was used to simulate the diesel autoignition on the RCM. The simulation results are found to agree well the experimental measurements over the whole temperature ranges. Species concentration analysis and brute force sensitivity analysis were also conducted to identify the crucial species and reactions controlling the autoignition of the diesel fuel.     

Autoignition delay modulation by high-frequency thermoacoustic oscillations in reheat flames

This paper investigates the sensitivity of the autoignition delay in reheat flames to acoustic pulsations associated with high-frequency transverse thermoacoustic oscillations. A reduced order model for the response of purely autoignition-stabilised flames to acoustic disturbances is compared with experimental observations. The experiments identified periodic flame motion associated with high-amplitude transverse limit-cycle oscillations in an atmospheric pressure reheat combustor. This flame motion was assumed to be the result of a superposition of two flame-acoustic coupling mechanisms: autoignition delay modulation by the oscillating acoustic field and displacement and deformation of the flame by the acoustic velocity. The reduced order model coupled to reaction kinetics calculations reveals that a significant portion of the observed flame motion can be attributed to autoignition delay modulation. The ignition position responds instantaneously to the acoustic pressure at the time of ignition, as observed experimentally. The model also provides insight into the importance of the history of acoustic disturbances experienced by the fuel-air mixture prior to ignition. Due to the high-frequency nature of the instability, a fluid particle can experience multiple oscillation cycles before ignition. The ignition delay responds in-phase with the net-acoustic perturbation experienced by a fluid particle between injection and ignition. These findings shed light on the underlying mechanisms of the flame motion observed in experiments and provide useful insight into the importance of autoignition delay modulation as a driving mechanism of high-frequency thermoacoustic instabilities in reheat flames.     

Stochastic simulation of variations in the autoignition delay time of premixed methane and air

A mesoscopic stochastic particle model for homogeneous combustion is introduced. The model can be used to investigate the physical fluctuations in a system of coupled chemical reactions with energy (heat) release/consumption. In the mesoscopic model, the size of the homogeneous gas volume is an additional variable, which is eliminated in macroscopic continuum models by the thermodynamic limit N→∞. Thus, continuous homogeneous models are macroscopic models wherein fluctuations are excluded by definition. Fluctuations are known to be of particular importance for systems close to the autoignition limits. The new model is used to investigate the stochastic properties of the autoignition delay time in a homogeneous system with stoichiometric premixed methane and air. Temperature and species concentrations during autoignition of sub-macroscopic volumes, including physically meaningful fluctuations, are presented. It is found that different realizations mainly differ in the time when ignition occurs; besides this the development is similar. The mesoscopic range and the macroscopic limit are identified. Which range a specific system is assigned to is not only a question of the length scale or particle number, but also depends on the complete thermodynamic state. The stochastic algorithm yields the correct results for the macroscopic limit compared to the continuous balance equations. The sensitivity of the results to two different detailed reaction mechanisms (for the same system) is studied and found to be low. We show that when approaching the autoignition limit by decreasing the temperature, the fluctuations in the autoignition delay time increase and an increasing number of realizations will have exceedingly long ignition delay times, meaning they are in practice not autoignitable. With this result the mesoscopic simulations offer an explanation of the transition between autoignitable and non-autoignitable conditions. The calculated distributions were compared with ten repetitions of the same experiment. A mesoscopic distribution that matches the experimental results was found.     

Autoignition in stratified mixtures for pressure gain combustion

The reliable generation of quasi-homogeneous autoignition inside a combustor fed by a continuous air flow would represent a milestone in realizing pressure gain combustion in gas turbines. In this work, the ignition distribution inside a stratified fuel–air mixture is analyzed. The ability of precise and reproducible injection of a desired fuel profile inside a convecting air flow is verified by applying tunable diode laser absorption spectroscopy in non-reacting measurements. High-speed, static pressure sensors and ionization probes allow for simultaneous detection of the flame and pressure rise at several axial positions in reactive measurements with dimethyl ether as fuel. A second, exchangeable combustion tube enables optical observation of OH* intensity in combination with pressure measurements. Experiments with three arbitrary fuel profiles show a set of ignition distributions that vary in shape, homogeneity, and the number of simultaneous autoignition events. Although the measurements show notable variation, a significant and reproducible influence of the fuel injection on the ignition distribution is observed. Results show that uniform autoignition leads to a coupling of the reaction front with the pressure rise and, therefore, induces a greater aerodynamic constraint than non-uniform ignition distributions, which are dominated by propagating deflagration fronts.     

Modeling evaporation effects in conditional moment closure for spray autoignition

Simulations of an n-heptane spray autoigniting under conditions relevant to a diesel engine are performed using two-dimensional, first-order conditional moment closure (CMC) with full treatment of spray terms in the mixture fraction variance and CMC equations. The conditional evaporation term in the CMC equations is closed assuming interphase exchange to occur at the droplet saturation mixture fraction values only. Modeling of the unclosed terms in the mixture fraction variance equation is done accordingly. Comparison with experimental data for a range of ambient oxygen concentrations shows that the ignition delay is overpredicted. The trend of increasing ignition delay with decreasing oxygen concentration, however, is correctly captured. Good agreement is found between the computed and measured flame lift-off height for all conditions investigated. Analysis of source terms in the CMC temperature equation reveals that a convective–reactive balance sets in at the flame base, with spatial diffusion terms being important, but not as important as in lifted jet flames in cold air. Inclusion of droplet terms in the governing equations is found to affect the mixture fraction variance field in the region where evaporation is the strongest, and to slightly increase the ignition delay time due to the cooling associated with the evaporation. Both flame propagation and stabilization mechanisms, however, remain unaffected.